It's a bit of a hassle, isn't it. I consider this one of perl's warts. You need a temporary to get the job done, which means you have to do something crufty like:

#! /usr/bin/perl -wl

use strict;

my @nums = qw/ 1 2 3 4 5 /;

my $sum = do { my $x; $x += $_ for @nums; $x };

print $sum;
[download]

... possibly localizing $_ in the do block. I sort of understand why things are the way they are, but I'm not good enough at internals to be able to explain in a clear manner :)